(CNN) Bewildered residents in rural Tennessee are grappling with fear and confusion as they try to understand why someone would send a bomb in the mail to their neighbor.
Retired lawyer John Setzer, 74, died Monday after “an unknown package exploded,” the Tennessee Bureau of Investigation said.
His wife, 72-year-old Marion Setzer, was seriously injured and airlifted to Vanderbilt University Medical Center.
Law enforcement sources say that a note was recovered from the bombing scene and was believed to have been attached to the bomb.
Investigators on Wednesday were cataloging evidence of a blast that was powerful enough to extend through much of the house and destroy windows.
A barrage of federal, state and local authorities descended on the neighborhood near Lebanon, Tennessee, about 30 miles east of Nashville. The FBI, U.S. postal inspectors, the Department of Homeland Security and the Bureau of Alcohol, Tobacco, Firearms and Explosives are also investigating.
So far, they haven’t publicly disclosed a motive. Those who knew the couple well are astonished.
“It doesn’t make sense at all,” family friend Ken Caldwell told CNN affiliate WTVF. “When I’ve heard it said that it was targeted, I thought, well, they must have targeted the wrong person.”
Michael Knight, spokesman for the ATF in Nashville, said investigators are looking at how the package was delivered and whether there were any threats to the family. They haven’t identified suspects, he said.
Investigators are testing items in the home, including labels and pieces of paper, to determine whether they were part of the package or perhaps previous deliveries.
One bomb expert, Joseph Vince, a former ATF agent, said investigators will looking at whether revenge may be a motive.
“Clients that didn’t like the way they were represented — or they represented some other client — now this is payback,” said Vince, director of criminal justice programs at Mount St. Mary’s University in Maryland.
“With a note in there, it very well may be some intentional revenge or something, that the attacker, the bomber, wanted to send them a message,” Vince added. “This is a very important piece of evidence, because now you may have handwriting.”
The victims took the package from the mailbox about 200 yards away from the house, which is in a rural area, Knight said. The blast occurred near the front door of the house, though it’s not clear whether just inside or right at the door.
Authorities said they haven’t seen any copy-cat incidents or secondary suspicious packages, which often happens in these cases.